Graphite-alumina-silicon carbide base refractory

ABSTRACT

This invention provides a graphite-alumina-silicon carbide base refractory having good erosion resistance, spalling resistance and durable for oxidizing attack at elevated temperature. This refractory contains more than 85 wt. percent of main components of graphite (including combined carbon), alumina and silicon carbide (the weight ratio of graphite:alumina:silicon carbide being 10-38:60-80:2-18), and these three main components are combined with each other by a carbon bond which contains glassy material (2-6 wt. percent) and forms the network structure.

United States Patent [191 Komaru et al.

GRAPHlTE-ALUMlNA-SILICON CARBIDE BASE REFRACTORY lnventors: lsamuKomaru; Kenzo Takeda;

Kazuo Yuki, all of Osaka, Japan Assignee: Nippon Crucible Co., Ltd.,Tokyo,

Japan Filed: Dec. 27, 1971 App]. No.: 212,394

US. Cl 106/56, 106/44, 106/65 Int. Cl C04b 35/52 Field of Search 106/44,56, 65

References Cited UNITED STATES PATENTS 1/1938 White ..106/44 11/1947Geiger 106/44 [451 Aug. 21, 1973 3,227,566 1/1966 Hilton et a1 106/443,329,514 7/1967 Saunders et al 106/44 Primary Examiner-James E. PoerAttorney-Richard C. Sughrue, J. Frank Osha et al.

[57] ABSTRACT This invention provides a graphite-alumina-silicon carbidebase refractory having good erosion resistance, spalling resistance anddurable for oxidizing attack at elevated temperature. This refractorycontains more than 85 wt. percent of main components of graphite(including combined carbon), alumina and silicon carbide (the weightratio of graphite:aluminazsilicon carbide being 1038:60-80:2-l8), andthese three main components are combined with each other by a carbonbond which contains glassy material (2-6 wt. percent) and forms thenetwork structure.

5 Claims, 1 Drawing Figure GRAPHITE-ALUMINA-SILICON CARBIDE BASEREFRACTORY DETAILED EXPLANATION OF THE INVENTION This invention relatesto a graphite-alumina-silicon carbide base refractory containing thegraphite, the alumina and the silicon carbide mutually combined by acarbon bond having a continuous network structure and a glassycomponent, and exhibiting excellent erosion resistance, spallingresistance and oxidation resistance at high temperature, as above l,000C.

The high alumina refractory produced by the electrocasting or burningprocess has such various favorable features as high refractory property,high load softening temperature, high mechanical strength, high abrasionresistance and high durability for erosive slag attacks, and accordinglyis applied to a high temperature furnace zone exposed to the relativelyshort range, for example, the lining of a rotary kiln or a glass meltingfurnace. This high alumina refractory, however, does not necessarilyexhibit the fully desirable spalling resistance when applied to furnaceparts which are exposed to the wide range by the intermittent attacks ofmolten metals or fused slags, for example, the mouth of a basic oxygenfurnace, the bottom plate of an upper pouring ingot mold or the tappingchannel of a blast furnace.

On the other hand, a graphite-silicon carbide refractory is easilyoxidized at high temperature in spite of the fact that it has a highpyrometric cone equivalent, high load softening point, chemicalneutrality, excellent erosion resistance due to poor wettability tomolten metals and slags, and strong resistance to abrupt temperaturechanges derived from high thermal conductivity.

After studies on both the erosive attacks of molten metals and slags onthe refractory and the thermal conductivity of the refractory relatingto the spalling resistance, the inventors have discovered that theerosion of relatively large sized particles in the refractorycomposition due to the attack of fused metals and slags is relativelyslow since large sized particles have small specific surface areas,whereas the erosion of small sized particles and the refractory matrix,which combines the particles with each other, is first caused to projectand slip out the coarse particles from the surface of the refractorybody as the coarse particles are kept uneroded.

Therefore, this invention was made on a discovery that the increase ofthe erosion resistance of the refractory matrix and the decrease of thesurface areas of the refractory matrix improve the erosion resistance ofthe refractory as a whole.

When the refractory is exposed to abrupt temperature change, the thermalstress is accumulated in the refractory whereby the cracks designated asthermal spalling are caused in the refractory. If the thermalconductivity of the refractory is high, the temperature difference inthe refractory is minimized and the thermal stress is not caused in therefractory, thus increasing the resistance to thermal spalling. Thethermal conductivity of the usual refractory is determined by itsporosity and the thermal conductivity of each constituent of therefractory, but this is not the case with the refractory of thisinvention having the continuous network of high thermally conductivematerial for bonding the refractory particles. That is to say, thethermal conductivity of this refractory is increased not only by thehighly conductive refractory particles themselves but also the highlyconductive binder network. According to empirical work, in the case thata network structure of highly thermal conductive material composed ofultra fine particles of graphite-silicon carbide and combined carbonhaving a thermal conductivity of 20 K cal/m. hr. C is applied to highlypure alumina coarse particles having a thermal conductivity of 3.5 Kcal/m.hr. C, the thermal conductivity of the resulting refractory doesnot increase remarkably if the content of the thermally conductivematerial is about 10 wt. percent, but in creases abruptly if the contentbecomes 20 wt. percent or more. The thermal conductivity of theresulting refractory almost accords with that of the thermal conductivematerial if the content of the latter reaches 50 wt. percent.

Therefore, the high alumina refractory of this invention has excellenterosion and spalling resistance and exhibits durability for oxidizingattacks at high temperature.

The action or function of each component of this refractory and thereason for determining the preferable ranges of the content of thecomponents will be described below.

The alumina, which may be obtained from the electrofusing or sinteringprocess, preferably has a purity of at least 94 wt. percent, and servesto increase the pyrometric cone equivalent, the load softening point,the mechanical strength, and the resistance to the erosive slag attacksfor the resulting refractory. The thermal conductivity of the purealumina is the highest among other components of the refractory, exceptfor the graphite and the silicon carbide. The alumina particles arepreferably composed of most parts of the coarse particles having a sizeof 4,760-297 microns and small parts of fine particles having a size ofless than 105 microns, but the grain size distribution is not restrictedto this range. particles having a size of The total amount of aluminalies preferably in 60-8 wt. percent. If the amount of alumina exceedswt. percent, the spalling resistance is worsened, whereas if the aluminaamount is not more than 60 percent, the erosion resistance, the fireresistance and the oxidation resistance are reduced. The electrofusedalumina particles having a rugged shape are well entangled with thenetwork structure of the carbon bond, and impart high mechanicalstrength to the resulting refractory. On the other hand, burned aluminaparticles having the fine pores or voids also impart high mechanicalstrength to the resulting refractory, as the carbon bond penetrates intothe pores.

The graphite, preferably natural graphite crystal, may have any shape offlake, vein or amorphous, so long as its ash content is not too much,but too large sized crystals can not be dispersed uniformly in thematrix, whereas too small sized crystals reduce the oxidation resistanceof the resulting refractory due to the increase of the surface areas ofthe graphite crystals. Therefore, the favorable grain size of thegraphite particle is that which passes through a sieve opening of 297microns.

If the graphite amount is not more than 8 wt. percent the desiredthermal conductivity, erosion resistance and wettability to'fused slagsor metals are not obtained, whereas if the graphite content exceeds 37wt. percent the desired oxidation resistance and mechani cal strengthare not obtained. The graphite content is explained as the carboncontent in the chemical analy- The silicon carbide disperses in thenetwork structure of the carbon bond and improves the oxidationresistance and the mechanical strength of the resulting refractory. Ifthe silicon carbide content is not more than 2 wt. percent, theaforesaid effect of the Si carbide is not observed, whereas if the Sicarbide content exceeds more than 18 wt. percent, the thermalconductivity and the erosion resistance are reduced.

If silicon or silicon alloy in pulverized form is added to the carbonbond, the mechanical strength of the resulting refractory is increaseddue to a formation of Si carbide by a reaction of Si or Si alloy withcarbon at relatively low temperature. The Si carbide thus formed, orthat added to the refractory composition from the first, is convertedinto SiO when brought into contact with oxygen at high temperature, andthis $10 sticks to the surface of the refractory body as a thin vitreousfilm which turns out to serve to prevent the oxidation of graphite andcombined carbon. The addition of less that 1 percent of Si or Si alloyto the carbon bond is ineffective, whereas the addition of more than 7percent of Si or Si alloy much reduces the fire resistance of theresulting refractory.

The forming and burning processes for the usual carbon bond refractorycan be applied to the production of the refractory of this invention.

The pitch or tar serves as the binder for the aforesaid components inthe forming or molding stage, but serves as the secondary bindertogether with the Si carbide and forms the aforesaid thermal conductivematerial after the forming is burned in the reducing atmosphere and thevolatile matters are expelled from the pitch or tar.

An excess amount of pitch or tar unwillingly increases the porosity ofthe resulting refractory and decreases the mechanical strength afterburning and the oxidation resistance, whereas an insufficient amount ofpitch or tar makes difficult the forming or molding of the startingcomposition.

Now, some examples of this invention will be described with reference tothe following Tables and an accompanied drawing which shows a ternarydiagram of graphite alumina and silicon carbide.

The sum of the amounts of alumina, carbon (sum of the graphite and thecombined carbon) and Si carbide must be at least 85 wt. percent afterthe burning of the starting refractory composition in a reducingatmosphere, and besides must lie in a hatched zone surrounded by point I(A1 80 wt. percent, C 18 wt. percent, SiC 2 wt. percent), point 2 7 11,080 percent, C 10 wt. percent, SiC 10 wt. percent, SiC 10 wt. percent),point 3 (A1 0 72 wt. percent, C 10 wt. percent, SiC 18 wt. percent),point 4 (A1 0, 60 wt. percent, C 22 wt. percent, SiC 18 wt. percent) andpoint (A1 0 60 wt. percent, C 38 wt. percent, SiC 2 wt. percent) in thedrawing.

Table 1 shows the starting composition of the refractory (A,B,C) of thisinvention.

The starting composition (A,B,C) was heated at a temperature 100-300" Chigher than the softening point of the pitch and well mixed at thattemperature. Then the heated composition was compressed and formed intothe desired shape with about 500-l,000 Kg/cm pressure in a molepreheated at 80-100 C, and successively burned at about l,300 C in areducing atmosphere.

TABLE 1 Percent A B C Electroeast alumina powder -4760 297 size) 48 50Electrocast alumina powder (105 size) 20 10 Sinternd alumina powder(4760 297 .1 size) 5O Sintered alumina powder (105 1 size) 8 Naturalgraphite powder (297 4 size) 18 29 20 Silicon carbide powder 7 4 l1Silicon powder 4 5 Ferrosilieon powder 4 Glassy material (.\I.P. about900 3 5 4 Pitch 4 4. 5 4 Tar 4 4. 5 4

Amounts of pitch and tar are indicated as weight parts with respect to100 weight parts of the refractories (A, B, C).

Table 2 shows the various physical properties of the burned refractories(A,B,C) of this invention, that of the conventional high aluminarefractory (D) and that of the conventional graphite-silicon carbiderefractory TABLE 2 A H (I l) l;

Apparent specific gravity. 3.261 1Z1 3. 21 l. 41 2.61 llnlk density.2.81 2.76 2. 77 5.17 2. 2 1 Apparunt porosity (percent) 14. 1 H1. 714.11 1.111 17.0 Compressive strength (kw/12111. (i261 3411 560 760 5X11Bl'll'llllK strength (kg/11111.

At room temp. 247 205 225 3111 220 At 1,200 0. 102 112 nut 100 105 At1,4(}0 C 8% 7-7 42 Load softening point T 1" (J.) (n I 1 Thermalr-xpanslnn rat (percent) at 1,60?

Above 1,700. Above 1,600.

Table 3 shows the results of the chemicalanalysis for the refractories(A,B,C,D,E).

TABLE 3 A B C D E SiO; 5.8 4.5 5.6 14.8 16.2 Al O, 67.8 57.5 58.1 89.9[.0 F; 1.5 3.0 1.8 3.1 3.8 CaO 0.6 0.8 0.7 0.6 0.5 SiC 7.5 5.1 12.3 47.1C 20.1 29 21.0 31.4 (wt.%)

The sum of3 main components, carbon, alumina and Si carbide was 95.1percent for A, 91.6 percent for B and 91.4 percent for C, respectively.The weight ratio of C: A1 0 SiO in the refractories A,B,C are shown inTable 4.

TABLE 4 A B c c 21.2 31.7 230 mp 71.0 62.7 63.3 $10 7.8 5.6 13.7

These C: A1 0 SiO ratios are shown in the drawing as point A, point Band point C.

The thermal conductivity, the spalling resistance and the oxidationresistance of refractories A-E are shown in Table 5.

The spalling resistance is tested by heating a refractory specimen at1,300 C, dipping the heated specimen immediately after removal from theheating furnace into water, and repeating the heating and water coolingcycle until crackformation is observed.

The oxidation resistance is tested by measuring a temperature at whichthe weight loss of the refractory specimen stops. The measuring of theweight loss is carried out by the thermobalance method.

TABLE 5 A B C D E Thermal conductivity (K cal/m.hrC) l3 l6 3.5 18Spalling resistance (times required for forming the first crack 4 5 5 l6 Oxidation stopping temp. (C) 12501240 I200 1 I60 The thermalconductivity, the spalling resistance and the oxidation resistance ofthe refractory specimens A, B and C are well comparative with that ofthe graphitesilicon carbide refractory specimen E, and therefore therefractory of this invention is well durable for the abrupt change oftemperature as similarly as the graphite-silicon carbide refractory.

The erosion resistances of the refractory specimens A-E were tested bythe following manner. Pluralities of refractory rod specimens having atrapezoid cross section were applied on the inner side wall of aninclined rotary tube furnace so that the inner face was formed as apolygonal tube. Metal or slag was charged in the rotating furnace, andan oxygen-acetylene flame was injected thereto to melt the metal orslag. After a predetermined period of time, the amount of erosion on theinner face of the polygonal tube was measured. Steel, pig iron,converter slag having a basicity of about 3 and blast furnace slaghaving a basicity of about 1.2

were used as the erosive agent for the refractory specimens. The testresults are shown in Table 6.

TABLE 6 A B C D E Steel 0.5 0.6 7 0.5 23 pig iron 0.4 0.3 0.4 l 8Converter slag 7 3 8 8 12 Blast furnace slag 4 4 3 5 5 The erosionamount of each specimen was set on the basis of the erosion amount ofspecimen D with pig iron.

It is apparent from the above Table 6 that refractory specimens A and Bare durable for the attack of molten steel and basic slag and suitablefor building steel making furnaces, and thatrefractory specimen C isdurable for the attack of molten pig and blast furnace slag and suitablefor building the blast furnace. The eroded surfaces of refractoryspecimens A, B and C were very smooth and not stuck by the metal andslag. Besides, these refractory specimens exhibited excellent spallingresistance and oxidation resistance at a high temperature range.

While this invention has been described with reference to particularembodiments thereof, it will be understood that the numerousmodifications may be made by those skilled in theart without actuallydeparting from the scope of the invention.

Therefore, the appended claims are intended to cover all such equivalentvariations as coming within the true spirit and scope of the invention.

What is claimed is:

1. A refractory consisting essentially of 10 to 38 percent by weight, asanalyzed carbon content, of graphite, 60 to percent by weight ofalumina, 2 to 18 percent by weight of silicon carbide, and a carbonbinder of a network structure containing 2 to 6 percent by weight of avitreous matter and combining said three ingredients with each other,the total amount of graphite including combined carbon, alumina, andsilicon carbide being at least percent of the total amount of therefractory.

2. The refractory of claim 1, wherein said alumina is an electrofused orsintered alumina having a purity of at least 94 percent.

3. The refractory of claim 1, wherein said graphite is a naturallyoccurring graphite in a flake, vein or amorphous form.

4. The refractory of claim 1, wherein said carbon binder contains 1 to 7percent by weight of silicon or its alloy in the pulverized state.

5. The refractory of claim 1, wherein said vitreous matter has asoftening point of 800 to l,200 C and has good wettability withgraphite.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 753,744 Dated August 21, 1973 In fl Isarnu Komaru et a1 It is certified thaterror appears in the above-identified pateht and that said LettersPatent are hereby corrected as shown below:

In The Heading:

Claim to Priority was omitted: Should Read;

Decenober 25,1970 Japan. 125620/70 Signed and sealed this 5th day ofMarch 1 71;.

( SEAL) Attest:

EDWARD M.FLETCI-IER,JR. I c. MARSHALL DANN I I Attesting OfficerCommissioner of Patents "ORM PO-1050H0-69) v i uscoMM-Dc 60376-P69 "fi'U.Si GOVERNMENT PRINTING OFFICE "I! O-J EI SSL I fl I

2. The refractory of claim 1, wherein said alumina is an electrofused orsintered alumina having a purity of at least 94 percent.
 3. Therefractory of claim 1, wherein said graphite is a naturally occurringgraphite in a flake, vein or amorphous form.
 4. The refractory of claim1, wherein said carbon binder contains 1 to 7 percent by weight ofsilicon or its alloy in the pulverized state.
 5. The refractory of claim1, wherein said vitreous matter has a softening point of 800* to 1,200*C and has good wettability with graphite.